The present invention relates to a portable telephone apparatus with camera, or in particular to a technique for the camera operating clock signal against the receiving frequency band in which the radio communication is conducted by the portable telephone apparatus.
The portable telephone apparatus includes a control circuit having a CPU (central processing unit) for controlling the whole system in addition to a radio circuit for conducting the radio communication. In recent years, the processing speed of the control CPU mounted on the portable telephone apparatus has been increased in keeping with the improved functions for games and mail. This has also increased the clock signal frequency for operation of the control CPU.
The clock signal input to the control CPU and the digital signal output in synchronism with the clock signal assume a rectangular waveform containing harmonic components as large as twice, thrice, quadruple, and so on of the output frequency. Each harmonic component has a characteristic of a smaller radiation energy, the larger the character n indicating the number of multiples of the harmonic component. The harmonic component has an effect as a radiation noise on the radio communication of the portable telephone apparatus.
With the recent trend toward an ever smaller size of the portable telephone apparatus, the control circuit such as the control CPU and the radio communication circuit having the function inherent to the portable telephone apparatus are arranged on a substrate integrated with or in proximity to each other. As a result, the radio communication circuit has come to be affected with a greater ease by the radiation noises of the harmonic components of the control CPU clock signal. In the case where a harmonic component of the clock signal of the control CPU coincides with the radio communication frequency band, the receiving sensitivity would be deteriorated extremely or the radio communication would otherwise be adversely affected.
The frequency used for the radio communication of the portable telephone apparatus, on the other hand, is configured of a plurality of frequency bands called channels. In accordance with the position and the radio communication condition of the portable telephone apparatus, the radio communication is carried out by switching the channel used from time to time in response to instructions from the radio base station. In order to prevent the radiation noise of the harmonic components from affecting the radio communication, therefore, it is necessary to design the apparatus in such a way that the channels used are not affected by the harmonic components.
In the case of the portable telephone apparatus using ARIB STD-T53, Band Class (JTACS Band) for the CDMA (Code Division Multiple Access) scheme, for example, the receiving frequency band is 832.0125 MHz to 869.9875 MHz. Also, the portable telephone apparatus uses the control CPU clock signal of about 20 MHz in frequency. In this case, the frequency 42 times as high as the harmonic component of 20 MHz is 840 MHz and the frequency 43 times as high as the harmonic component is 860 MHz, both of which cause an interference by being superposed on the receiving frequency band.
A conventional technique providing a protective measure against the problem of the interference caused by the harmonic components of the clock signal frequency described above includes a method in which the noise source is covered with a metal case and shielded electrically.
This technique is employed for a portable electronic device (TV or computer) having a liquid display device and a circuit board built therein, in which a molded member with a conductive plating is arranged between the liquid crystal display device and the circuit board as a noise shielding member. In this way, the radiation noises of harmonic components generated from the circuit board are reduced (JP-A-6-301448).
In another conventional technique, the harmonic components of the control CPU clock signal are prevented from interfering with the radio communication channels and thus the noises in the radio communication are reduced by changing the operating clock signal frequency of the devices in accordance with the frequency band of the radio communication channels.
This technique is used with a portable telephone apparatus for conducting the radio communication with a plurality of communication channels, comprising a memory unit, a clock signal generating means and a control unit operated by the clock signal, wherein the frequency of the clock signal is regulated by a setting for changing the clock frequency not to interfere with the communication channels stored in advance in the memory unit based on the communication channel information. In this way, the interference is eliminated by preventing the harmonic components of the operating clock signal from being superposed on the frequency band of the communication channels (JP-A-7-303079).
In recent years, the portable telephone apparatus market has seen the increased sales of the portable telephone apparatuses with camera. The portable telephone apparatus with camera has gained the popularity due to its ability to pick up an image at any desired place and send it immediately to a receiver by electronic mail.
On the other hand, to meet the strong demand for a high pixel density, portable telephone apparatuses with camera having a pixel density as high as 1M or 2M pixels have recently been placed on the market.
In addition, the user demands a smooth image in monitoring and a smooth dynamic image that has been picked up. For a smooth image to be obtained, the number of image frames output from the camera within a predetermined time length (frame rate) is required to be increased.
The increased pixel density and the increased frame rate have led to an increased data rate of the image output from the camera and a higher rate of the camera operating clock signal. Specifically, the camera operating clock is set to a higher frequency than the clock signal frequency of the control circuit in some cameras placed on the market.
Many image pickup devices of the camera in the portable telephone apparatus now employ the CCD (charge coupled device) for its high sensitivity, high S/N and the ease with which to increase the pixel density.
The CCD is a charge transfer device capable of moving the charge as a mass of signals sequentially at the rate synchronized with the operating clock signal pulses derived from an external source. The CCD requires a voltage of about +15 V or −7.5 V at the time of charge transfer, and vertical transfer pulses or signal read pulses of about +15 V, −7.5 V and 22.5 V in potential difference for driving the CCD camera are generated in synchronism with the operating clock signal.
On the other hand, the voltage used in the control circuit of the portable telephone apparatus is about 3 V or 1.8 V, and therefore the radiation energy of the harmonic components of the camera is approximately five times higher than that of the control circuit. With the increase in the operating clock signal rate of the camera of the portable telephone apparatus, therefore, the problem of interference by the radiation noises of the harmonic components becomes more conspicuous. It is of urgent necessity to take an appropriate measure against this problem.
The manner in which the radiation noises of the harmonic components interfere with the receiving channel is shown in FIG. 3. The operating clock signal frequency of the camera is given as fc, which is accompanied by harmonic of 2fc, 3fc, and so forth. The explanation below assumes that the receiving band is 832.0125 MHz to 869.9875 MHz and fc=32 MHz. FIG. 3 indicates that the frequency of 864 MHz about 27 times as high as the camera operating frequency fc of 32 MHz interferes with the receiving band.
FIG. 4 is an enlarged view of the receiving band used in the conventional radio communication. This receiving band includes a plurality of receiving channels A to F each assumed to have a channel width of about 1.2 MHz by way of explanation.
The radio communication of the portable telephone apparatus is carried out using one of a plurality of channels (A to F) specified by a common carrier in compliance with an instruction from a radio base station in accordance with the radio communication conditions. In the case where the channel D is used in compliance with an instruction of the radio base station, for example, the channel D is superposed on the frequency of 864 MHz which is a harmonic 27 times as high as the camera operating frequency fc of 32 MHz, and the resultant interference of the radiation noise of the harmonic component poses the problem of a deteriorated receiving sensitivity.
The CCD camera, which has a module structure configured of a CCD sensor and a lens, requires a focal length and hence a predetermined height. The height of about 10 mm is required for the ¼-inch 1.3 M-pixel CCD sensor, for example.
In order to keep up with the trend toward a thinner body of the portable telephone apparatus, the CCD sensor is not mounted on but connected to the circuit board through a flexible circuit. In this case, the radiation noise of the harmonic components of the clock signal for driving the CCD camera is released from the flexible circuit and has a greater effect on the radio communication.
In the conventional electrical shielding method described in JP-A-6-301448, the modules and even the flexible circuit of the camera are required to be shielded and therefore a compact, lightweight shield case cannot be fabricated. Further, the cost is increased by an amount corresponding to the shield case. Thus, this conventional method is not suitable for the portable telephone apparatus.
The prior art described in JP-A-7-303079, on the other hand, in which the clock signal frequency of the control circuit such as a control CPU is selectively changed, fails to deal with the operating clock signal frequency of the camera. In the application to the portable telephone apparatus with camera, therefore, the problem is posed that the interference cannot be reduced against the radiation noise of the harmonic components generated from the camera having a large radiation energy.